Method for manufacturing a substrate

ABSTRACT

A method for manufacturing a substrate includes the following steps: (a) providing a support substrate with a first coefficient of thermal expansion, having on one of its faces a first plurality of trenches parallel to each other in a first direction, and a second plurality of trenches parallel to each other in a second direction; (b) transferring a useful layer from a donor substrate to the support substrate, the useful layer having a second coefficient of thermal expansion; wherein an intermediate layer is inserted between the front face of the support substrate and the useful layer, the intermediate layer having a coefficient of thermal expansion between the first and second coefficients of thermal expansion.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.15/743,004, filed Jan. 9, 2018, now U.S. Pat. No. 10,943,778 issued Feb.17, 2021, which is a national phase entry under 35 U.S.C. § 371 ofInternational Patent Application PCT/EP2016/066609, filed Jul. 13, 2016,designating the United States of America and published as InternationalPatent Publication WO 2017/012940 A1 on Jan. 26, 2017, which claims thebenefit under Article 8 of the Patent Cooperation Treaty to FrenchPatent Application Serial No. 1501519, filed Jul. 17, 2015, thedisclosure of each of which is hereby incorporated herein in itsentirety by this reference.

TECHNICAL FIELD

The present disclosure relates to a method of manufacturing a substratecomprising a support substrate having a first coefficient of thermalexpansion (CTE) and a useful layer having a second coefficient ofthermal expansion different from the first coefficient of thermalexpansion.

BACKGROUND

Referring to FIG. 1 , the manufacture of acoustic surface wave filters(F-SAW) may comprise the formation of interdigitated metal combelectrodes 5 on one side of a piezoelectric material, for example,lithium tantalate LiTaO₃.

The geometrical characteristics of the inter-digitated comb electrodes,such as the comb size, or their spacing, determine the resonantfrequency and the quality factor of the SAW filters.

In this respect, a person skilled in the art can refer to the article“Recent development of temperature compensated SAW devices,” Ken-yaHashimoto et al., IEEE Ultrasonics Symposium (IUS), 2011 IEEEInternational, pp. 79-86.

In normal use, the filters may be subjected to temperatures in the rangeextending from −40° C. to 85° C.

However, piezoelectric materials such as lithium tantalate have acoefficient of thermal expansion on the order of 14 in at least one ofthe crystallographic directions.

Consequently, the temperature variations induce a variability of thegeometric characteristics of the F-SAW filters.

To overcome this problem, it is possible to position a stiffenersubstrate on one side of the piezoelectric material.

FIG. 2 shows a structure comprising a silicon substrate 1, a layer ofLiTaO₃ 3 formed on the silicon substrate 1, and interdigitated combmetal electrodes 5 on the free surface 4 of the LiTaO₃ layer 3.

The low coefficient of thermal expansion of silicon, the latter beingequal to 2.6×10⁻⁶/° C., makes it possible to limit the expansion of theLiTaO₃ layer and consequently to limit the variations of the resonancefrequency and the quality factor of the F-SAW filter. This effect isreferred to hereinafter as “thermal compensation.”

The method of manufacturing a substrate comprising a layer of LiTaO₃ ona silicon substrate involves the following manufacturing steps:

-   -   a. providing a support substrate 1, for example, of silicon; and    -   b. the transfer of a layer of LiTaO₃ 3 on one of the faces of        the support substrate 1, specifically the front face 1 a.

The transfer of the LiTaO₃ layer is generally carried out by assemblinga LiTaO₃ substrate with a silicon substrate, followed by a step ofthinning, for example, mechanical thinning, of the LiTaO₃ substrate.

This method is, however, not satisfactory because it requires astrengthening of the bonded interface between the LiTaO₃ and the siliconby way of a heat treatment.

Indeed, the difference between the thermal expansion coefficients ofLiTaO₃ and silicon causes a degradation of the bonding interface, namelya detachment and/or the appearance of cracks in the LiTaO₃ layer.

US Patent Publication No. 2009/0267083 A1 proposes to overcome theaforementioned drawbacks by providing a trench network on the rear faceof the support substrate 1. The formation of the trenches makes itpossible to limit the constraints at the bonding interface and, thus,ensures the integrity of the useful layer.

This solution is not satisfactory because it does not accommodate largedifferences in thermal expansion coefficient.

BRIEF SUMMARY

An object of the present disclosure is, therefore, to provide a methodof manufacturing a substrate adapted to withstand heat treatment whileretaining the thermal compensation effect.

The disclosure solves the technical problem and relates to a method ofmanufacturing a substrate comprising a useful layer placed on areceiving substrate, the method comprising the following steps:

-   -   a. Providing a support substrate with a first coefficient of        thermal expansion, having on one of its faces, referred to as        the front face, a first plurality of trenches parallel to each        other in a first direction, and a second plurality of trenches        parallel to each other in a second direction not parallel to the        first direction; and    -   b. Transferring a useful layer from the donor substrate to the        support substrate, the useful layer having a second coefficient        of thermal expansion different from the first coefficient of        thermal expansion.

The method further includes insertion of an intermediate layer betweenthe front face of the support substrate and the useful layer, theintermediate layer having a coefficient of thermal expansion between thefirst and second coefficients of thermal expansion.

Therefore, the presence of the first plurality of trenches and thesecond plurality of trenches allows one to break the constraint fieldexerted on the interface between the intermediate layer and the supportsubstrate.

Furthermore, the presence of the intermediate layer enables limiting theeffect of the difference between the first and second coefficients ofthermal expansion, while maintaining a thermal compensation effect.

In addition, the presence of the intermediate layer enables having aspace between the trenches of each of the pluralities of trenchesgreater than the dimension of an F-SAW intended to be formed on thesubstrate thus obtained.

According to one embodiment, step b. of transferring the useful layercomprises the assembly of the donor substrate with the supportsubstrate, and the thinning of the donor substrate in order to form theuseful layer.

According to one embodiment, the thinning of the donor substrate iscarried out using mechanical thinning.

According to one embodiment, the intermediate layer is formed, beforestep b. of transferring the useful layer, onto the support substrate oronto the donor substrate.

According to one embodiment, the intermediate layer comprises a glassmaterial.

According to one embodiment, the intermediate layer comprises at leastone of the materials included in the group consisting of TEOS, BPSG,PSG, and USG.

According to one embodiment, the trenches of the first plurality oftrenches are regularly arranged every 3 to 10 millimeters.

According to one embodiment, the trenches of the second plurality oftrenches are regularly arranged every 3 to 10 millimeters.

According to one embodiment, the trenches of the first plurality oftrenches and the second plurality of trenches have a depth between 1 and100 μm.

According to one embodiment, the trenches of the first plurality oftrenches and the second plurality of trenches have a width between 1 and100 μm.

According to one embodiment, the useful layer comprises at least one ofthe materials included in the group consisting of LiTaO₃ and LiNbO₃.

According to one embodiment, the difference between the firstcoefficient of thermal expansion and the second coefficient of thermalexpansion is greater than 5×10⁻⁶/° C., and preferably greater than10×10⁻⁶/° C.

According to one embodiment, the useful layer comprises at least one ofthe materials included in the group consisting of silicon, germanium,silicon carbide, alumina, sapphire, or aluminum nitride.

Additional embodiments of the present invention include a substratecomprising:

-   -   a. a support substrate with a first coefficient of thermal        expansion, having on one of its faces, referred to as the front        face, a first plurality of trenches parallel to each other in a        first direction, and a second plurality of trenches parallel to        each other in a second direction not parallel to the first        direction; and    -   b. a useful layer with a second coefficient of thermal expansion        different from the first coefficient of thermal expansion and        arranged on the support substrate.

The substrate further includes an intermediate layer inserted betweenthe front face of the support substrate and the useful layer, theintermediate layer having a coefficient of thermal expansion between thefirst and second coefficients of thermal expansion.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be better understood in light of the followingdescription of specific and non-limiting example embodiments of theinvention with reference to the accompanying figures (drawings) inwhich:

FIG. 1 illustrates interdigitated combs forming a filter with surfaceacoustic waves according to previously known methods;

FIG. 2 illustrates a substrate used for producing a filter with surfaceacoustic waves according to previously known methods; and

FIG. 3 is a schematic illustration of an embodiment of a method formanufacturing according to the present disclosure.

DETAILED DESCRIPTION

For the various embodiments, the same references will be used foridentical elements or ensuring the same function, in the interests ofsimplification of the description.

The method according to the disclosure comprises a step a. of providinga support substrate 10 with a first coefficient of thermal expansion.

The support substrate 10 can comprise at least one of the materialschosen from the group consisting of silicon, germanium, silicon carbide,alumina, sapphire, and aluminum nitride.

The support substrate 10 exhibits a coefficient of thermal expansionbetween 2×10⁻⁶/° C. and 9×10⁻⁶/° C.

For example, a support substrate 10 comprising silicon exhibits acoefficient of thermal expansion of 2×10⁻⁶/° C.

The support substrate 10 also comprises on one of its faces, that is,the front face 11, a first plurality of trenches 12. The trenches of thefirst plurality of trenches 12 are parallel to each other in a firstdirection.

The support substrate 10 can also comprise, on its front face 11, asecond plurality of trenches 13. The trenches of the second plurality oftrenches 13 are parallel to each other in a second direction. The seconddirection is not parallel to the first direction.

The trenches of the first plurality of trenches 12 can be regularlyarranged every 3 to 10 millimeters.

The trenches of the second plurality of trenches 13 can be regularlyarranged every 3 to 10 millimeters.

The trenches of the first plurality of trenches 12 can have a depthbetween 1 and 100 μm.

The trenches of the second plurality of trenches 13 can have a depthbetween 1 and 100 μm.

The trenches of the first plurality of trenches 12 and the secondplurality of trenches 13 can have a width between 1 and 100 μm.

The trenches of the first plurality of trenches 12 and of the secondplurality of trenches 13 can be formed by a sawing wheel of the typegenerally used in the microelectronic industry. The sawing techniquesusing such a sawing wheel are well known in the art.

The width of the trenches of the first plurality of trenches 12 and thesecond plurality of trenches 13 is defined by the thickness of thesawing wheel.

In addition, the depth of the trenches is also defined by the sawingdepth.

The trenches of the first and second pluralities of trenches can also beachieved by the formation of an etching mask on the front face 11 of thesupport substrate 10. The aforementioned etching mask designs the shapeof the trenches of the first plurality of trenches 12 and the secondplurality of trenches 13, on the front face 11 of the support substrate10.

The etching forming the trenches is, therefore, executed by using asuitable etching.

For example, in embodiments in which the support substrate 10 is asilicon substrate, the etching mask may comprise silicon dioxide, andthe etching may be carried out using a KOH solution.

After the forming of the trenches of the first plurality of trenches 12and of the second plurality of trenches 13, an intermediate layer 20 isformed on the front face 11 of the support substrate 10.

The intermediate layer 20 can conformally cover the front face 11 of thesupport substrate 10 by conforming to the shape of the trenches.

The intermediate layer 20 can have a thickness comprised between 0.5 and50 μm.

Advantageously, the intermediate layer 20 can deform itself plasticallywhen it is subjected to a thermal treatment with a rise in temperature.

Advantageously, the intermediate layer 20 can comprise a glass material.

For example, the intermediate layer 20 can comprise at least one of thematerials included in the group consisting of borophosphosilicate glass(BPSG), tetraethyl orthosilicate oxide (TEOS oxide), phosphosilicateglass (PSG), and undoped silicate (USG).

The method according to the disclosure also comprises a step b., whichcomprises the forming of a useful layer 31 on the intermediate layer 20.

The useful layer 31 can be formed by a method of layer transfer from adonor substrate 30.

The transfer step can comprise the assembly of a donor substrate 30 withthe intermediate layer 20.

The assembly step can be a step of bonding by molecular adhesion.

A step of thermal treatment can be carried out so as to reinforce thebonding interface.

For example, a thermal treatment can be carried out at a temperaturebetween 80° C. and 150° C., preferably between 100° C. and 120° C., fora period of time between 30 minutes and 4 hours, under a non-oxidizingatmosphere, for example, under nitrogen and/or argon and/or helium.

In order to reinforce the bonding interface, the donor substrate 30surface intended to be put into contact with the intermediate layer 20can be activated with plasma.

For example, the activation can be carried out with a dioxygen plasma(O₂) or with dinitrogen (N₂), for 30 seconds, at a power output of 500Watts, and a pressure of 50 mTorr.

The assembly step is, therefore, followed by a thinning of the donorsubstrate 30. Therefore, the thinned donor substrate 30 forms the usefullayer 31.

The thinning of the donor substrate 30 can be executed by usingtechniques well known in the art, such as mechanical thinning, and/or bymechanical-chemical polishing.

The useful layer 31 can, therefore, following the thinning step of thedonor substrate 30, have a thickness between 5 and 50 μm.

According to a variant of the method of manufacturing according to thedisclosure, the intermediate layer 20 can be formed on the donorsubstrate 30. In these conditions, the assembly step comprises puttingin contact the intermediate layer 20 with the front face 11 of thesupport substrate 10.

The useful layer 31 has a second coefficient of thermal expansiondifferent from the first coefficient of thermal expansion.

The useful layer 31 can comprise a perovskite-type material.

The useful layer 31 can comprise a dielectric material, the materialpreferably being a ferroelectric material.

The useful layer 31 can comprise at least one of the materials includedin the group consisting of LiTaO₃ and LiNbO₃.

According to the disclosure, the intermediate layer 20 has a coefficientof thermal expansion between the first coefficient of thermal expansionand the second coefficient of thermal expansion.

The difference between the first coefficient of thermal expansion andthe second coefficient of thermal expansion may be greater than 5×10⁻⁶/°C. and, preferably, greater than 10×10⁻⁶/° C.

For such differences in thermal expansion coefficients, the methodaccording to the disclosure enables ensuring the integrity of thepackage comprising the support substrate 10, the intermediate layer 20and the useful layer 31 at the time of thermal annealing.

In this way, the presence of the intermediate layer 20 enables limitingthe effect of the difference between the first coefficient of thermalexpansion and the second coefficient of thermal expansion, whilemaintaining a thermal compensation effect.

In addition, the presence of at least the first plurality of trenches 12on the front face 11 of the support substrate 10 enables disrupting thecontinuity of the constraints present at the interface formed by theintermediate layer 20 and the front face 11.

Therefore, taken in combination, the effect of the intermediate layer 20and the presence of at least the first plurality of trenches 12 enableslimiting the constraints exerted on the interfaces formed on the onehand, by the useful layer 31 and the intermediate layer 20, and on theother hand, by the intermediate layer 20 and the support substrate 10.

First Embodiment

According to a first embodiment, the disclosure comprises the provisionof a silicon support substrate 10.

The first plurality of trenches 12 are formed with a sawing wheel. Thetrenches of the first plurality of trenches 12 are spaced from eachother by 3 mm, have a depth of 100 μm, and a width of 100 μm.

The intermediate layer 20, which is formed directly on the front face 11of the support substrate 10, comprises borophosphosilicate glass, andits thickness is between 0.5 and 50 μm, for example, 2 μm.

The intermediate layer 20 conforms to the topography of the front face11 of the support substrate 10.

Still according to the first embodiment, a donor substrate 30 of LiTaO₃is provided.

The surface of the donor substrate 30 intended to be placed in contactwith the intermediate layer 20 is activated by plasma comprisingdioxygen.

The surface of the activated donor substrate 30 is, therefore, assembledwith the intermediate layer 20 by molecular adhesion.

The assembly interface formed by the donor substrate 30 and theintermediate layer 20 is reinforced by a thermal annealing carried outat 100° C. for a period of 3 hours under an argon atmosphere.

The donor substrate 30 is then thinned mechanically to a thickness of 20μm, in order to form the useful layer 31.

Second Embodiment

The second embodiment differs from the first embodiment in that theintermediate layer 20 is formed on the donor substrate 30 rather than onthe front face 11 of the support substrate 10 and in that the plasmaactivation is achieved on the front face 11 of the support substrate 10.

What is claimed is:
 1. A method for manufacturing a substrate comprisinga useful layer on a receiving substrate, the method comprising:providing a support substrate having a first coefficient of thermalexpansion; providing a donor substrate comprising at least one materialselected from the group consisting of LiTaO₃ and LiNbO₃, the at leastone material having a second coefficient of thermal expansion differentfrom the first coefficient of thermal expansion; providing anintermediate layer on a surface of at least one of the support substrateand the donor substrate, the intermediate layer having a coefficient ofthermal expansion between the first coefficient of thermal expansion andthe second coefficient of thermal expansion; assembling the donorsubstrate with the support substrate with the intermediate layerdisposed between the donor substrate and the support substrate; andtransferring a useful layer of the at least one material from the donorsubstrate onto the support substrate, the intermediate layer disposedbetween the useful layer and the support substrate.
 2. The method ofclaim 1, wherein transferring the useful layer comprises thinning thedonor substrate to form the useful layer.
 3. The method of claim 2,wherein thinning the donor substrate comprises mechanical thinning thedonor substrate.
 4. The method of claim 3, further comprising providingthe intermediate layer on the surface of the at least one of the supportsubstrate and the donor substrate before transferring the useful layerfrom the donor substrate onto the support substrate.
 5. The method ofclaim 1, wherein the support substrate comprises, on a front facethereof: a first plurality of trenches parallel to each other in a firstdirection; and a second plurality of trenches parallel to each other ina second direction not parallel to the first direction.
 6. The method ofclaim 5, wherein the trenches of the first plurality of trenches areseparated from one another by an average distance of from 3 millimetersto 10 millimeters.
 7. The method of claim 6, wherein the trenches of thesecond plurality of trenches are separated from one another by anaverage distance of from 3 millimeters to 10 millimeters.
 8. The methodof claim 5, wherein the trenches of the first plurality of trenches andthe second plurality of trenches have an average depth between 1 μm and100 μm.
 9. The method of claim 8, wherein the trenches of the firstplurality of trenches and the second plurality of trenches have anaverage width between 1 μm and 100 μm.
 10. The method of claim 1,wherein providing an intermediate layer on a surface of at least one ofthe support substrate and the donor substrate comprises depositing aglass on the surface of at least one of the support substrate and thedonor substrate.
 11. The method of claim 10, wherein the glass comprisesat least one material selected from the group consisting of TEOS, BPSG,PSG, or USG.
 12. The method of claim 1, wherein the difference betweenthe first coefficient of thermal expansion and the second coefficient ofthermal expansion is greater than 5×10-6/° C.
 13. The method of claim12, wherein the difference between the first coefficient of thermalexpansion and the second coefficient of thermal expansion is greaterthan 10×10-6/° C.
 14. The method of claim 1, wherein the supportsubstrate comprises at least one material selected from the groupconsisting of silicon, germanium, silicon carbide, alumina, sapphire,and aluminum nitride.
 15. A substrate, comprising: a support substratehaving a first coefficient of thermal expansion, the support substrateincluding, on a front face thereof: a first plurality of trenchesparallel to each other in a first direction; and a second plurality oftrenches parallel to each other in a second direction not parallel tothe first direction; a useful layer having a second coefficient ofthermal expansion different from the first coefficient of thermalexpansion, the useful layer comprising at least one material selectedfrom the group consisting of LiTaO₃ and LiNbO₃; and an intermediatelayer disposed between the support substrate and the useful layer, theintermediate layer having a coefficient of thermal expansion between thefirst coefficient of thermal expansion and the second coefficient ofthermal expansion.
 16. The substrate of claim 15, wherein theintermediate layer comprises a glass.
 17. The substrate of claim 16,wherein the glass comprises at least one material selected from thegroup consisting of TEOS, BPSG, PSG, or USG.
 18. The substrate of claim15, wherein a difference between the first coefficient of thermalexpansion and the second coefficient of thermal expansion is greaterthan 5×10-6/° C.
 19. The substrate of claim 15, wherein the supportsubstrate comprises at least one material selected from the groupconsisting of silicon, germanium, silicon carbide, alumina, sapphire,and aluminum nitride.
 20. A surface acoustic wave (SAW) device,comprising: a substrate, including: a support substrate having a firstcoefficient of thermal expansion, the support substrate including, on afront face thereof: a first plurality of trenches parallel to each otherin a first direction; and a second plurality of trenches parallel toeach other in a second direction not parallel to the first direction; auseful layer having a second coefficient of thermal expansion differentfrom the first coefficient of thermal expansion, the useful layercomprising at least one material selected from the group consisting ofLiTaO₃ and LiNbO₃; and an intermediate layer disposed between thesupport substrate and the useful layer, the intermediate layer having acoefficient of thermal expansion between the first coefficient ofthermal expansion and the second coefficient of thermal expansion; andelectrodes on or in the useful layer of the substrate.